Many conventional methods exist for forming seals which prevent leakage of liquid, gas, and the like from one section of a package to another. For example, conventional methods for forming seals entail: (a) using O-rings; (b) welding; (c) soldering; and (d) gluing. Many such conventional methods are acceptable in environments where sealing is not very sensitive or important. However, in many environments, including those where sensors are placed along different sections of a package to measure pressure, obtaining an airtight or hermetic seal is important and conventional methods are not adequate.
In typical conventional pressure sensor devices, a convoluted, thin, metal diaphragm is welded in a pressure path to seal (so as to reduce leakage of gas from) a gas-filled chamber. In manufacturing other types of conventional pressure sensor devices, a metal housing is fabricated using stainless steel alloys (due to their anti-corrosion characteristics). In such cases, a sealing method typically entails the use of a laser beam or an electron beam (“e-beam”) to melt the metal to form a hermetic seal.
A MEMS (“Micro-Electro-Mechanical Systems”) pressure sensor device uses a silicon base which is mounted on a metal, for example, Kovar or Invar, whose coefficient of thermal expansion closely matches that of the silicon base (in general, stainless steel alloys cannot be used). Due to continuing requirements for low cost pressure sensor devices, it is desirable that a metallic housing for such devices be made from materials such as brass, copper and aluminum, and not stainless steel. However, joining thermally matched metal (for example, Kovar Invar) to a metallic housing (made, for example, of thermally mismatched metals such as brass, copper or aluminum) using a conventional laser welding method typically results in weld cracks. In other words, conventional laser welding methods used with these metals have been unsuccessful in forming hermetic seals. Further, alternate sealing methods such as brazing and soldering require processes at temperatures above 800° C., which high temperatures are unacceptable for use in fabricating MEMS sensor devices.